Core Unit B will provide Program Project Grant (PPG) investigators with three services: high precision whole-body in vivo neutron activation (IVNA) analysis measurements of H, C, N, O, Ca, P, Na, and C1, and whole-body counting for K; expert assistance with interpretation and analysis of IVNA results; and physics consultations for new instrument design and development. Instrumentation for high precision measurements of body composition by IVNA were developed and equipped with state-of-the-art hardware during PPG I. Subsequent improvements in the analytical strategies extended the range of subjects that were included in the studies during PPG II measurement precisions of the IVNA facilities were further improved using new and improved phantoms and by implementing a Monte Carlo code (MCNP) to carry out extensive simulations for different body configurations and stature. During this period extensive calibrations and cross-calibrations with investigators at Baylor for Ca and with investigators at Core Unit A for total body potassium (TBK), were carried out with publication of results. Replacement of older computers with newer systems enabled transfer from an early software environment to contemporary operating systems. These improvements along with basic operations upgrades allowed measurement routinely of up to six patients per week with stabilized systems. Core support during the proposed PPG III for clinical research projects will continue and important new initiatives will be instituted, including: new calibrations for measuring TBK and Ca in obese patients in Project 3; correlation of partial/regional body nitrogen to total body nitrogen with Core A; creation of independent MCNP input files that describe the human body as simulated by a BOMAB phantom for each of the three activation facilities; and design of a Gamma Nuclear Resonance Absorptiometry system for elemental imaging in vivo. The Core's activities are consistent with the Department of Energy's mandate to develop novel technologies, to transfer these systems to cooperating laboratories following their developing and testing phase, and to maintain state-of-the art systems for use as national and internal reference standards.